1. Field of the Invention
The present invention relates to a printing apparatus and a printing method for printing an image on a printing medium by actuating a printing head such as an inkjet printing head under its scanning movement.
2. Description of the prior Art
FIG. 2 shows an example of a printing apparatus that can be constructed as a main part of an inkjet color printer or the like. For printing an image on a sheet of printing paper 105 being placed on a platen 106, at first, a driving motor 103 is activated to drive a driving belt 109 by which a carriage 102 is shifted to a position facing a home-position sensor 108 in the direction of main-scanning (i.e., the direction perpendicular to the direction of feeding the sheet of printing paper). Then, the carriage 102 moves forward in a scanning movement in the direction of the arrow A to bring printing heads 120, 121, 122, and 123 toward a predetermined scanning area. These printing heads 120-123 are respectively provided for ejecting black (K), cyan (C), magenta (M), and yellow (Y) color inks. These color inks are ejected onto a sheet of printing paper 105 to make an image while the carriage travels through the predetermined scanning area. The forward-scanning movement of the carriage 102 is stopped when the image printing of a predetermined length is terminated. Subsequently, the carriage 102 starts to move in the reverse direction toward the position facing the home-position sensor 108 as a reverse-scanning movement thereof in the direction of the arrow B. During the reverse-scanning movement, a paper-feed motor 107 drives a paper-feed roller 105 to feed the paper in the direction of the arrow C (i.e., sub-scanning direction). Repeating the cycle of these steps, the printing of a color image on the printing paper 105 can be completed. In the figure, by the way, the reference numeral 100 denotes a second paper-feed roller and 111 denotes a sensor for detecting the presence or absence of paper on the platen 106.
Referring now to FIG. 3, there is shown the relationship between a speed of the carriage 102 that moves in the direction of forward-scanning and an interval of time required for printing a line of image. In the figure, the motor 103 is activated at a point of time indicated by the reference numeral 130 to move the carriage 102 in the direction of forward scanning. During the period T1 (i.e., acceleration time), the carriage 102 is accelerated. After the point of time 131 at which the carriage 102 reaches a predetermined speed, the printing heads 120, 121, 122, and 123 start ink ejection respectively to form an image on a sheet of paper. At the point of time 132 after lapse of the time T2, the printing movement is terminated and the carriage 102 is decelerated. Finally, the carriage 102 comes to a stop at the point of time 133 after lapse of the time T3. Accordingly, the printing movement of the conventional printing apparatus requires several steps as described above because of the impossibility of an increase in the rotational speed of the motor right up to the printing speed of the carriage 102. Furthermore, the printing speed is also defined by the printing resolution and the refill frequency. In this description, by the way, the term xe2x80x9cprinting speedxe2x80x9d means a speed of the carriage during the interval of ejecting ink from the printing heads; and the term of xe2x80x9crefill frequencyxe2x80x9d means a number of times each of the printing heads 120, 121, 122, and 123 is refilled with ink after ejecting ink within a specified interval.
The printing speed of the carriage 102 can be calculated, for example, by the following equation (1).
V=(25.4/R)xc3x97Fxe2x80x83xe2x80x83(1),
wherein xe2x80x9cRxe2x80x9d denotes a printing resolution (dots per inch); xe2x80x9cFxe2x80x9d denotes a refill frequency (10 kHz); xe2x80x9cVxe2x80x9d denotes a printing speed (millimeter per second); and xe2x80x9c25.4xe2x80x9d is a scale factor (i.e., one inch is equal to 25.4 millimeters).
If xe2x80x9cRxe2x80x9d=600 dpi and xe2x80x9cFxe2x80x9d=10 kHz, for example, then the printing speed xe2x80x9cVxe2x80x9d can be calculated using the above equation (1) as follows.
V=(25.4/R)xc3x97F=(25.4/600)xc3x9710000=423.33 (mm/s).
In this case, therefore, the carriage 102 shifts its position at that speed. A linear encoder (not shown) optically or magnetically recognizes the scanning position of the carriage 102. Thus, the printing heads eject ink droplets with reference to output signals from the linear encoder, resulting in an image formed by equally placing the ink dots on a sheet of the printing paper 105. Accordingly, the above description facilitates the understanding of the need for the intervals of time for acceleration and deceleration of the carriage 102 to attain the formation of equally distributed ink dots.
FIG. 4 illustrates the example of ejecting ink from the printing head at the time of accelerating and decelerating the carriage 102. In the figure, a solid line 300 indicates the relationship between the carriage""s speed and time just as is the case with FIG. 3. Encoder pulses 301 are generated from the linear encoder (not shown) that optically or magnetically recognizes the scanning position of the carriage 102. If the printing head ejects an ink droplet by the falling edge of an encoder pulse, a dot to be formed on a sheet of the printing paper 105 can be represented by the reference numeral 302, as schematically shown in FIG. 4. During the intervals of accelerating and decelerating the carriage 102, as can be seen from FIG. 4, dots are not equally distributed on the printing paper 105. This means that a locus of a flying ink droplet is changed with respect to the scanning speed of the carriage 102. That is, an ink droplet ejected from the printing head reaches a point which is displaced a distance xe2x80x9cX1xe2x80x9d from a predetermined point in the direction of carriage travel. The distance xe2x80x9cX1xe2x80x9d can be expressed by the following equation (2).
X1=VCr1xc3x97(S/V)xe2x80x83xe2x80x83(2),
wherein, xe2x80x9cSxe2x80x9d denotes a distance between the printing head and a sheet of the printing paper 105 (see FIG. 5A); xe2x80x9cVxe2x80x9d denotes a speed of an ink droplet ejected from the printing head (see FIG. 5B); and xe2x80x9cVCrlxe2x80x9d denotes a speed of the carriage that travels in the direction of forward-scanning (see FIG. 5B).
According to the equation (1), as shown in FIG. 5C, the deviation xe2x80x9cX1xe2x80x9d doubles (i.e., 2xc3x97X1=X2) as the carriage speed xe2x80x9cVCr1xe2x80x9d doubles (i.e., 2xc3x97VCr1=VCr2). Therefore, the conventional printing apparatus must start the printing after the carriage attains a constant speed and also controls the carriage so as to be kept at a constant speed during the step of printing an image on the printing paper.
Regarding the movement of the carriage 102 during the step of printing, the conventional example described above requires both acceleration and deceleration times T1, T3 in addition to the actual printing time T2, so that the conventional approach takes a long time to complete the entire process, resulting in difficulty of attaining the high-speed printing movement. It means that a needless or wasted time (T1+T3) is required for printing a band (i.e., an amount of image which can be printed by one scanning movement of the carriage). If the number of the scanning movements of the carriage 102 to be required for printing a page (i.e., one complete image to be printed on one side of a sheet of paper) is xe2x80x9cNxe2x80x9d, there is a needless time xe2x80x9c(T1+T3)xc3x97Nxe2x80x9d in addition to an actual printing time xe2x80x9cT2xc3x97Nxe2x80x9d. In this case, furthermore, attention must be directed toward additional spaces extending in the directions of both forward and reverse movements of the carriage, respectively. Such spaces are required for both the acceleration and deceleration movements by the time xe2x80x9cT1+T3xe2x80x9d. Consequently, due to such additional spaces, the width of the printing apparatus becomes large.
The conventional printing apparatus has another disadvantage in that a quality of the image may decline as a result of variations in the spaces between dots printed on the printing paper when a variation in the speed of the carriage mechanically occurs in spite of printing an image only in the phase of moving the carriage at a constant speed.
An object of the present invention is to provide a printing apparatus that allows high-speed printing of a high quality image on a printing medium without a variation in the scanning speed of a printing head and a method for printing a high quality image on a printing medium using such a novel printing apparatus.
According to one aspect of the present invention, a printing apparatus, for printing an image on a printing medium on the basis of image data by actuating a printing head during a scanning movement of the printing head, includes detecting means and compensating means. The detecting means detects a scanning speed of the printing head. The compensating means establishes the amount of deviation with respect to timing for actuating the printing head in response to the scanning speed detected by the detecting means.
According to another aspect of the present invention, a method, for printing an image on a printing medium on the basis of image data by actuating a printing head during a scanning movement of the printing head, includes the steps of detecting a scanning speed of the printing head and establishing the amount of deviation with respect to timing for actuating the printing head in response to the scanning speed detected in the detecting step.
The present invention is able to correct the timing of activating a printing head (e.g., inkjet printing head) in response to a scanning speed thereof, so that high quality image formation with equally distributed pixels such as ink dots on a printing medium can be attained whether or not variations in a scanning speed of the printing head are generated.
The present invention also allows high quality image formation whether or not a variation of the scanning speed occurs at the time of moving the printing head at a predetermined constant speed.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.